It is standard to form a three-dimensional workpiece of a thermoplastic resin by first heating an appropriately shaped mold, then applying a fusible powder, for instance of polyethylene, to the heated mold surface until it melts, and then cooling the mold and fused layer of resin thereon and stripping it from the mold. To ensure that the surface of the molded piece is smooth it is standard to subject it to a reflowing operation in which the surface of the hardened workpiece is heated sufficiently to melt it and allow it to flow slightly.
Such reflowing is typically carried out by passing the finished workpiece through a tunnel-type oven. Air is heated in this oven and directed over the workpiece as it moves through, melting the surface and causing it to flow smooth. The substantial disadvantage of this system is that the oven is very bulky and consumes a substantial amount of energy. In addition dust-and moisture-carrying air are drawn into the open ends of the oven and particles and droplets can get stuck to the finished workpiece, ruining it.